Sunday, December 30, 2012

Islands in the Intertidal

Note: This program first aired December 15, 2012.

If the intertidal zone can be considered one of the most rigorous habitats on the planet, due to its consistent inconsistency, part time ocean, part time land, then tide pools are the oasises in the intertidal zone’s deserts.  A tide pools is like an island. An island is a little piece of land, terrestrial environment, surrounded by water, a physiologically hostile environment (at least to the land based organism on the island). A tide pool is the same thing in reverse—an oasis of habitat stranded in a larger hostile environment.

As a general concept, tide pools allow organisms to live further away from the contiguous coastal ocean than they “normally” would; further away meaning higher up the intertidal zone. The pools provide a little piece of sea like habitat in the middle of a dry beachfront or rocky shore. Of course, all tide pools are not created the same, and it would be a misconception to think that they perfectly recreate the conditions found subtidally. No, tide pools, though they do offer marine organisms a more desirable situation than simply being stranded on a rock in the sun at low tide, do present their own set of physiologic challenges and inconsistencies to the plants and animals that call them home.

The further away a tide pool is from the low tide line, the less like the ocean it tends to be. And the smaller or lower volume a tide pool is, the more easily its environmental parameters can stray from those found in the mother ocean as well. The shape of the tide pool matters too—a big wide but shallow tide pool may have a high volume of water but that water will have a wide surface area in contact with both the atmosphere and the underlying substrate, both vectors for change of the physical parameters of the pool. Finally, the tidal cycle itself varies. Sometimes of the month the tide is low during the hottest sunniest part of the day, at other parts of the monthly tidal cycle, the intertidal zone is covered during this time. The organisms that live in this environment must be endlessly and admirably flexible, dealing with dramatically different parameters on a daily basis.

All of this is to illustrate the main point, that though tidepools are typically characterized by their height in the intertidal zone, no two tidepools are the same, and the high and low characterizations I’m about to talk about need to be taken with a grain of sea salt.

The higher the tide pool is in the intertidal zone, the further it is from the contiguous coastal ocean and the longer the organisms that live in it are castaways “on the island”. The temperature of these pools can vary greatly, as air temperatures vary much more than ocean temperatures due to the differences in the physical properties of air and water. In winter these pools get colder than the ocean, in summer they get much warmer. The salinity can vary much more than the ocean as well; when it rains or snows, the salinity in a high tidepool can drop dramatically as the sea water is diluted by fresh water precipitation. When the sun is intense and the water is easily heated, salinities can increase as evaporation takes place. Even the pH of a high tidepool varies. At  night the pool can experience low pHs (acid conditions) because the organisms in the pool are respiring and producing carbon dioxide, which when dissolved in water, creates carbonic acid. During the day, the photosynthetic organisms in the tide pool will photosynthesize and take up carbon dioxide, which causes the pH to increase and the water to become less acidic. High tide pools tend to have lower biological diversity because of the wide range in physical parameters.

Tide pools that are closer to the ocean, have less variation in all those parameters, because they are replenished by the contiguous coastal ocean sooner and more often than the higher tidepools. As a result, the lower tidepools exhibit higher biodiversity and host communities that look more like neighboring subtidal zones. The closer the island is to shore, the more like shore it will look.